Particle beam devices are used for examining samples (objects) in order to obtain insights with regard to the properties and behavior of the samples under specific conditions. One type of particle beam device is an electron beam device, in particular a scanning electron microscope (also known as SEM).
In an SEM, an electron beam (primary electron beam) is generated using a beam generator. The electrons of the primary electron beam are accelerated to a predeterminable energy and focused by a beam guiding system, in particular an objective lens, onto a sample to be analyzed (i.e., an object to be analyzed). A high-voltage source having a predeterminable acceleration voltage is used for acceleration purposes. Using a deflection unit, the primary electron beam is guided in a raster-type fashion over a surface of the sample to be analyzed. In this case, the electrons of the primary electron beam interact with the material of the sample to be analyzed. In particular, interaction particles and/or interaction radiation arise(s) as a consequence of the interaction. By way of example, electrons are emitted by the sample to be analyzed (so-called secondary electrons) and electrons of the primary electron beam are backscattered at the sample to be analyzed (so-called backscattered electrons). The secondary electrons and backscattered electrons are detected and used for image generation. An image of the sample to be analyzed is thus obtained.
Furthermore, it is known from the prior art to use combination devices for processing and/or for analyzing a sample, wherein both electrons and ions can be guided onto a sample to be processed and/or to be analyzed. By way of example, it is known for an SEM to be additionally equipped with an ion beam column. Using an ion beam generator arranged in the ion beam column, ions are generated which are used for processing a sample (for example for removing a layer of the sample or for depositing material on the sample, wherein the material is provided by a gas injection system) or else for imaging. In this case, the SEM serves, in particular, for observing the processing, but also for further analysis of the processed or non-processed sample. Electrons may also be used for depositing material. This is known as Electron Beam Induced Deposition (EBID).
It is known in the prior art to use a so-called Everhart-Thornley detector (ETD) for detecting the above-mentioned interaction particles, namely the secondary electrons and/or backscattered electrons in an SEM. The ETD comprises an extraction grid, a scintillator and a light detector. The secondary electrons or backscattered electrons are attracted away from the sample to the ETD by the extraction grid and are accelerated to the scintillator having a high voltage of about 10 kV. When the highly kinetic electrons impinge on the scintillator, photons are generated which are detected by the light detector, for example a photomultiplier.
The ETD is used when the ambient pressure of the ETD is lower than or equal to 10−3 hPa. The ambient pressure is, for example, the pressure in an object chamber of a particle beam device in which a sample to be analyzed is arranged. For example, the ambient pressure of the ETD is at high vacuum, which is the pressure range of 10−3 hPa to 10−7 hPa. When the ambient pressure of the ETD is above 10−3 hPa, increased conductivity of residual gas leads to overstrikes (hereinafter also called breakdowns) due to the high voltage applied to the scintillator.
Therefore, such an ETD may not be used in a particle beam device in which the ambient pressure of the ETD may be varied to a pressure of higher than 10−3 hPa. Such a particle beam device is also known as a variable pressure particle beam device, for example a variable pressure SEM.
It is known to use an indirect detection of the secondary electrons at ambient pressures above 10−3 hPa in an object chamber of a particle beam device. The indirect detection comprises applying an extraction potential of up to 1000 V to an electrode in order to accelerate the secondary electrons toward the electrode and away from the sample. A charged particle cascade in the form of an to electron cascade results from collisions of the secondary electrons with gas molecules of a gas, for example ambient air in the object chamber. Tertiary electrons arise in this charged particle cascade, and photons are also generated from scintillation effects. Signal detection takes place either by the measurement of the electron current of the tertiary electrons or by the detection of the generated photons.
A particle beam device is known which may be operated in a first operation mode and in a second operation mode. In the first operation mode, the particle beam device is operated at ambient pressures in the object chamber equal to or lower than 10−3 hPa, for example at high vacuum conditions or ultra-high vacuum conditions. In the second operation mode, the particle beam device is operated at variable pressure conditions in the object chamber, i.e. at ambient pressures in the object chamber equal to or above 10−3 hPa. However, the known particle beam device has to have different detectors for the first operation mode and for the second operation mode. Therefore, the known particle beam device comprises a first detector used in the first operation mode and a second detector used in the second operation mode. Having different detectors increases the costs of such a particle beam device. Additionally, the different detectors require a certain amount of space in the particle beam device which might otherwise be used for other units of the particle beam device or which could be saved. Moreover, since the different detectors may not have an identical orientation towards the sample to be analyzed, errors might occur in the analysis of the sample.
A detector for an SEM has been developed to avoid the necessity to use two different detectors in the two different operation modes mentioned above. The known detector detects both electrons and light. The detector comprises a collector grid, a scintillator and a light detector, wherein the scintillator is of a material transmissive for visible light arranged in front of the light detector. The scintillator may be provided with a coating transparent to visible light. In the first operation mode, the operation of the known detector is analogous to the mode of operation of an ETD. A high potential between 5 kV and 15 kV is applied to the scintillator so that the high-energy secondary electrons and backscattered electrons impinge on the scintillator and generate photons in the scintillator. The photons are detected by the light detector. In the second operation mode, a potential between 50 V and 1000 V is applied to the collector grid and/or the scintillator. The secondary electrons or backscattered electrons generated by the interaction of the primary particle beam with the sample generate an electron cascade in a gas with scintillation effects on the path from the sample to the scintillator or collector grid. The photons generated in the electron cascade pass through the scintillator and are detected by the light detector. Although the known detector offers a detection of charged particles in the first operation mode as well as in the second operation mode, the use of such a detector may be disadvantageous since the detection principles in the first operation mode and in the second operation mode differ from each other such that errors might occur in the analysis of the sample to be analyzed. Moreover, the known detector may suffer from reduced performance in the first and second operation mode compared to using two dedicated detectors for the two operation modes.
As regards prior art, reference is made in particular to patent documents U.S. Pat. No. 4,785,182 A, WO 98/22971 A2 as well as U.S. Pat. No. 7,462,839 B2, both of which are incorporated by reference herein.
In light of the aforesaid, it is desirable to provide a mechanism that detects charged particles with the same detection principle when the ambient pressures in the object chamber are equal to or above 10−3 hPa as well as lower than 10−3 hPa.